The p21 Cdk-interacting protein Cip1 is a potent inhibitor of G1 cyclin-dependent kinases

Mitogen-activated protein kinases (MAPKs) are important signal transducing enzymes, unique to eukaryotes, that are involved in many facets of cellular regulation. Initial research concentrated on defining the components and organization of MAPK signalling cascades, but recent studies have begun to shed light on the physiological functions of these cascades in the control of gene expression, cell proliferation and programmed cell death.

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Mammalian MAP kinase signalling cascades

The evolutionarily conserved checkpoint protein kinase, TOR (target of rapamycin), has emerged as a major effector of cell growth and proliferation via the regulation of protein synthesis. Work in the last decade clearly demonstrates that TOR controls protein synthesis through a stunning number of downstream targets. Some of the targets are phosphorylated directly by TOR, but many are phosphorylated indirectly. In this review, we summarize some recent developments in this fast-evolving field. We describe both the upstream components of the signaling pathway(s) that activates mammalian TOR (mTOR) and the downstream targets that affect protein synthesis. We also summarize the roles of mTOR in the control of cell growth and proliferation, as well as its relevance to cancer and synaptic plasticity.

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Upstream and downstream of mTOR

Mitogen-activated protein (MAP) kinases comprise a family of ubiquitous proline-directed, protein-serine/threonine kinases, which participate in signal transduction pathways that control intracellular events including acute responses to hormones and major developmental changes in organisms. MAP kinases lie in protein kinase cascades. This review discusses the regulation and functions of mammalian MAP kinases. Nonenzymatic mechanisms that impact MAP kinase functions and findings from gene disruption studies are highlighted. Particular emphasis is on ERK1/2.

/pdf/mitogen-activated-protein-map-kinase-pathways-regulation-and-3nsauvepr2.pdf

Mitogen-activated protein (MAP) kinase pathways: regulation and physiological functions.

The transmission of extracellular signals into their intracellular targets is mediated by a network of interacting proteins that regulate a large number of cellular processes. Cumulative efforts from many laboratories over the past decade have allowed the elucidation of one such signaling mechanism, which involves activations of several membranal signaling molecules followed by a sequential stimulation of several cytoplasmic protein kinases collectively known as mitogen-activated protein kinase (MAPK) signaling cascade. Up to six tiers in this cascade contribute to the amplification and specificity of the transmitted signals that eventually activate several regulatory molecules in the cytoplasm and in the nucleus to initiate cellular processes such as proliferation, differentiation, and development. Moreover, because many oncogenes have been shown to encode proteins that transmit mitogenic signals upstream of this cascade, the MAPK pathway provides a simple unifying explanation for the mechanism of action...

https://faseb.onlinelibrary.wiley.com/doi/pdf/10.1096/fasebj.9.9.7601337

The MAPK signaling cascade.

MAP kinase (relative molecular mass, 42,000), a low abundance serine--threonine protein kinase, is transiently activated in many cell types by a variety of mitogens, including insulin, epidermal growth factor, and phorbol esters. In vitro, MAP kinase will phosphorylate and reactivate S6 kinase II previously inactivated by phosphatase treatment. Because many of the stimuli that activate MAP kinase are also stimulators of cell proliferation, and regulation of the cell cycle seems to involve a network of protein kinases, MAP kinase could be important in the transmission of stimuli eventually leading to the progression from G0 to G1 in the cell cycle. Activated MAP kinase contains both phosphotyrosine and phosphothreonine. We report here that MAP kinase can be deactivated completely by treatment with either phosphatase 2A, a protein phosphatase specific for phosphoserine and phosphothreonine, or CD45, a phosphotyrosine-specific protein phosphatase. We demonstrate that MAP kinase is only active when both tyrosyl and threonyl residues are phosphorylated and suggest therefore that the enzyme functions in vivo to integrate signals from two distinct transduction pathways.

Requirement for integration of signals from two distinct phosphorylation pathways for activation of MAP kinase

Ribosomal protein S6 is a component of the eukaryotic 40S ribosomal subunit that becomes phosphorylated on multiple serine residues in response to a variety of mitogens, including insulin, growth factors, and transforming proteins of many oncogenic viruses. Recently, an activated S6 kinase (S6 KII) has been purified to homogeneity from Xenopus eggs1, and characterized immunologically2 and at the molecular level3. Purified S6 KII can be deactivated in vitro by incubation with either protein phosphatase 1 or protein phosphatase 2A. Reactivation and phosphorylation of S6 KII occurs in vitro with an insulin-stimulated micro-tubule-associated protein-2 (MAP-2) protein kinase which is itself a phosphoprotein that can be deactivated by protein phosphatase 2A. These studies suggest that a step in insulin signalling involves sequential activation by phosphorylation of at least two serine/threonine protein kinases.

Insulin-stimulated MAP-2 kinase phosphorylates and activates ribosomal protein S6 kinase II

Purified maturation-promoting factor contains the product of a Xenopus homolog of the fission yeast cell cycle control gene cdc2+.

Cyclin is a component of maturation-promoting factor from Xenopus

Maturation-promoting factor causes germinal vesicle breakdown when injected into Xenopus oocytes and can induce metaphase in a cell-free system. The cell-free assay was used to monitor maturation-promoting factor during its purification from unfertilized Xenopus eggs. Ammonium sulfate precipitation and six chromatographic procedures resulted in a preparation purified greater than 3000-fold that could induce germinal vesicle breakdown within 2 hr when injected into cycloheximide-treated oocytes. Proteins of 45 kDa and 32 kDa were correlated with fractions of highest activity in both assays. These fractions contained a protein kinase activity able to phosphorylate the endogenous 45-kDa protein, as well as histone H1, phosphatase inhibitor 1, and casein. The highly purified preparations described here should help to identify the mechanism of action of maturation-promoting factor and to elucidate the role of protein kinases in the induction of metaphase.

/pdf/purification-of-maturation-promoting-factor-an-intracellular-out3bdd3wt.pdf

James L. Maller

Papers

Requirement for integration of signals from two distinct phosphorylation pathways for activation of MAP kinase

Insulin-stimulated MAP-2 kinase phosphorylates and activates ribosomal protein S6 kinase II

Purified maturation-promoting factor contains the product of a Xenopus homolog of the fission yeast cell cycle control gene cdc2+.

Cyclin is a component of maturation-promoting factor from Xenopus

Purification of maturation-promoting factor, an intracellular regulator of early mitotic events